The invention relates generally to torque control of a machine, and in particular to torque control of permanent magnet (PM) synchronous machines used in electric power steering systems.
PM synchronous motors are attractive as servo drives because of their high power densities. The stator phases of such motors may be electrically excited to produce a controlled torque on the rotor, the torque being proportional to the field intensity of the rotor magnets and the amplitude of the stator phase excitation, thus permitting control of the rotor motion. When such motors are employed for high performance servo drive applications, which require precise control of rotor motion, feedback signals representing rotor position and rotor velocity are used. These feedback sensors are often generated using current sensors connected to phase windings of a PM synchronous motor to achieve effective torque control. An exemplary system using current sensors for rotor position control is described in U.S. Pat. No. 5,569,994. The use of current sensors, however, adds expense to the overall system and tends to cause low frequency torque ripple. Accordingly, there is a need in the art for a method and system for providing torque control without the use of current sensors coupled to the phase windings.
An exemplary embodiment of the invention is a method for torque control of a PM synchronous machine. The method includes obtaining a torque command signal and a machine speed and determining an operating mode in response to the torque command signal and the machine speed. The operating mode includes a first operating mode and a second operating mode. In the first operating mode, a stator phase voltage magnitude is computed and an angle between the stator phase voltage and a stator phase back emf is determined in response to the stator phase voltage magnitude. In the second operating mode, the stator phase voltage is set to a predetermined magnitude and the angle between the stator phase voltage and the stator phase back emf is determined in response to the predetermined magnitude.